Photoelectric Effect

UXL Encyclopedia of Science
COPYRIGHT 2002 The Gale Group, Inc.

Photoelectric effect

When visible light, X rays, gamma rays, or other forms of electromagnetic radiation are shined on certain kinds of matter, electrons are ejected. That phenomenon is known as the photoelectric effect. The photoelectric effect was discovered by German physicist Heinrich Hertz (1857–1894) in 1887. You can imagine the effect as follows: Suppose that a metal plate is attached by two wires to a galvanometer. (A galvanometer is an instrument for measuring the flow of electric current.) If light of the correct color is shined on the metal plate, the galvanometer may register a current. That reading indicates that electrons have been ejected from the metal plate. Those electrons then flow through the external wires and the galvanometer, providing the observed reading.

Photoelectric theory

The photoelectric effect is important in history because it caused scientists to think about light and other forms of electromagnetic radiation in a different way. The peculiar thing about the photoelectric effect is the relationship between the intensity of the light shined on a piece of metal and the amount of electric current produced.

Words to Know

Anode: The electrode in an electrochemical cell at which electrons are given up to a reaction.

Cathode: The electrode in an electrochemical cell at which electrons are taken up from a reaction.

Electrode: A material that will conduct an electrical current, usually a metal, used to carry electrons into or out of an electrochemical cell.

Electromagnetic radiation: Radiation (energy in the form of waves or subatomic particles) that transmits energy through the interaction of electricity and magnetism.

Frequency: The number of times a wave passes a given point in space per unit of time (as per second).

Photocell: A vacuum tube in which electric current flows when light strikes the photosensitive (or light sensitive) cathode.

To scientists, it seemed reasonable that you could make a stronger current flow if you shined a brighter light on the metal. More (or brighter) light should produce more electric current—or so everyone thought. But that isn't the case. For example, shining a very weak red light and a very strong red light on a piece of metal produces the same results. What does make a difference, though, is the color of the light used.

One way that scientists express the color of light is by specifying its frequency. The frequency of light and other forms of electromagnetic radiation is the number of times per second that light (or radiation) waves pass a given point. What scientists discovered was that light of some frequencies can produce an electric current, while light of other frequencies cannot.

Einstein's explanation. This strange observation was explained in 1905 by German-born American physicist Albert Einstein (1879–1955). Einstein hypothesized that light travels in the form of tiny packets of energy, now called photons. The amount of energy in each photon is equal to the frequency of light (ν) multiplied by a constant known as Planck's constant (ℏ), or νℏ.

Einstein further suggested that electrons can be ejected from a material if they absorb exactly one photon of light, not a half photon, or a third photon, or some other fractional amount. Green light might not be effective in causing the photoelectric effect with some metals, Einstein said, because a photon of green light might not have exactly the right energy to eject an electron. But a photon of red light might have just the right amount of energy.

Einstein's explanation of the photoelectric effect was very important because it provided scientists with an alternative method of describing light. For centuries, researchers had thought of light as a form of energy that travels in waves. And that explanation works for many phenomena. But it does not work for phenomena such as the photoelectric effect and certain other properties of light.

Today, scientists have two different but complementary ways of describing light. In some cases, they say, it behaves like a wave. But in other cases, it behaves like a stream of particles—a stream of photons.

Applications

Two of the most important applications of the photoelectric effect are the photoelectric cell (or photocell) and solar cells. A photocell usually consists of a vacuum tube with two electrodes. A vacuum tube is a glass tube from which almost all of the air has been removed. The electrodes are two metal plates or wires. One electrode in a photocell consists of a metal (the cathode) that will emit electrons when exposed to light. The other electrode (the anode) is given a positive electric charge compared to the cathode. When light shines on the cathode, electrons are emitted and then attracted to the anode. An electron current flows in the tube from cathode to anode. The current can be used to turn on a motor, to open a door, or to ring a bell in an alarm system. The system can be made to respond to light, as described above, or it can be sensitive to the removal of light.

Photocells are commonly used in factories. Items on a conveyor belt pass between a beam of light and a photocell. As each item passes the beam, it interrupts the light, the current in the photocell stops, and a counter is turned on. With this method, the exact number of items leaving the factory can be counted. Photocells are also installed on light poles to turn street lights on and off at dusk and dawn. In addition, photocells are used as exposure meters in cameras. They measure the exact amount of light entering a camera, allowing a photographer to adjust the camera's lens to the correct setting.

Solar cells are devices for converting radiant energy (light) into electrical energy. They are usually made of specially prepared silicon that emits electrons when exposed to light. When a solar cell is exposed to sunlight, electrons emitted by silicon flow through external wires as a current.

Individual solar cells produce voltages of about 0.6 volts each. In most practical applications, higher voltages and large currents can be obtained by connecting many solar cells together. Electricity from solar cells is still quite expensive, but these cells remain very useful for providing small amounts of electricity in remote locations where other sources are not available. As the cost of producing solar cells is reduced, however, they will begin to be used for the production of large amounts of electricity for commercial use.

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photoelectric effect

The Columbia Encyclopedia, 6th ed.

Copyright The Columbia University Press

photoelectric effect, emission of electrons by substances, especially metals, when light falls on their surfaces. The effect was discovered by H. R. Hertz in 1887. The failure of the classical theory of electromagnetic radiation to explain it helped lead to the development of the quantum theory. According to classical theory, when light, thought to be composed of waves, strikes substances, the energy of the liberated electrons ought to be proportional to the intensity of light. Experiments showed that, although the electron current produced depends upon the intensity of the light, the maximum energy of the electrons was not dependent on the intensity. Moreover, classical theory predicted that the photoelectric current should not depend on the frequency of the light and that there should be a time lag between the reception of light on the surface and the emission of the electrons. Neither of these predictions was borne out by experiment. In 1905, Albert Einstein published a theory that successfully explained the photoelectric effect. It was closely related to Planck's theory of blackbody radiation announced in 1900. According to Einstein's theory, the incident light is composed of discrete particles of energy, or quanta, called photons, the energy of each photon being proportional to its frequency according to the equation E=hυ, where E is the energy, υ is the frequency, and h is Planck's constant. Each photoelectron ejected is the result of the absorption of one photon. The maximum kinetic energy, KE, that any photoelectron can possess is given by KE = hυ-W, where W is the work function, i.e., the energy required to free an electron from the material, varying with the particular material. The effect has a number of practical applications, most based on the photoelectric cell.

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photoelectric effect

photoelectric effect Liberation of electrons from the surface of a material when light, ultraviolet radiation, X-rays, or gamma rays fall on it. The effect can be explained only by the quantum theory: photons in the radiation are absorbed by atoms in the substance and enable electrons to escape by transferring energy to them.

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